Method and device for identifying double feeds

ABSTRACT

The invention relates to a method and a device for identifying overlap during the successive transport of flat mail in a double belt system. After edges have been detected on both sides of the transport path and after the distances between the detected edges and the respective guiding front edge measured by a light barrier have been determined, both sides of the longitudinal sides of the mail are acted upon with different transport speeds, ensuring that the minimum gaps are maintained. The distances between the edges and the guiding front edge are then re-measured. A double feed is detected if there is a change in distance.

[0001] The invention relates to a method of, and to an arrangement for, detecting double withdrawals according to the preambles of claims 1 and 6.

[0002] One problem associated with separating apparatuses resides in so-called double-withdrawal errors which arise when, in addition to the item of mail which is to be withdrawn from a stack, a further item of mail is also withdrawn. In distributing installations for flat items of mail, the double withdrawals which are not detected are particularly important since they increase the incorrect distributing rate due to reading errors in the automatic address recognition, but may also result in items of mail accumulating and being destroyed in consequence.

[0003] In the case of a known solution, moveable item parts are temporarily deflected perpendicularly to the conveying apparatus. By evaluating the spring-back behavior of the items of mail, the presence of overlapped items of mail is then detected (EP 0 650 911 B1). Since the overlapped items of mail may be offset to different extents, the deflection has to take place both on the front side and on the rear side of the items of mail, which results in increased outlay. The mechanical stressing to which the items of mail are subjected during deflection is also disadvantageous. A further drawback of this solution is that the extent to which the rear edges are offset must not fall below a minimum value (approximately 10 mm).

[0004] In another known solution, the profile of the height of the items of mail is measured and, in the case of an abrupt changing height, an overlap is signalled (DE 196 25 044 A1).

[0005] It is possible for two items of mail to overlap in ways which cannot be detected by the abovementioned solutions.

[0006] This is the case, in particular, in the case of identical items of mail lying congruently in relation to one another. It is also possible, however, for a largest item of mail to cover a smallest item of mail such that they are transported further, in the form of a double withdrawal, unnoticed.

[0007] The object of the invention is thus to provide a method and an arrangement which are intended for detecting overlaps of items of mail and with the aid of which the overlaps can be detected irrespective of the size of the items of mail and their position in relation to one another.

[0008] This object is achieved according to the invention by the features of patent claims 1 and 6.

[0009] Following determination of the front and rear edges of the items of mail and of the item gaps by means of light barriers, the following steps are carried out with the aid of the arrangement:

[0010] detecting edges on both sides of the transporting path and determining the spacings of the detected edges from the respective leading front edge measured by the light barrier (Li),

[0011] following the edge detection, subjecting the free longitudinal sides of the separated items of mail to frictional force, with different transporting speeds on both sides, in a transporting segment for item displacement, the selection of the differing transporting speeds taking place such that, in the case of overlap, the items of mail would be displaced in relation to one another such that the gap dimensions do not fall below a minimum value, and the action-specific lengths and points in time being selected such that only ever one item of mail is gripped on each side,

[0012] determining the edges on both item sides, the leading front edges and the spacings of the edges from the respective leading front edge once again following the displacement of the items of mail in relation to one another and detection of an overlap if at least one spacing between the mutually corresponding edges and the respective leading front edge or additional edges has changed in relation to the first measurements.

[0013] By twice measuring the spacings between the edges which are determined on both sides and the respective front edge before and after the items of mail are subjected to frictional force, with different transporting speeds on both sides, and the corresponding evaluation, it is possible, on the one hand, to distinguish edges produced by flaps or viewing windows from front edges of overlapped subsequent items of mail, and thus to avoid items of mail being separated out incorrectly, and, on the other hand, to detect as overlaps small items of mail which are concealed behind large items of mail and then to eject them in order to avoid, for example, incorrect sorting.

[0014] In order that the ejecting operation can take place quickly and reliably via a diverter arrangement, the overlapped items of mail here are displaced such that the necessary gap dimensions between items of mail do not fall below a minimum value.

[0015] Advantageous configurations of the invention are illustrated in the subclaims.

[0016] In order that individual items of mail run through the transporting segment for item displacement without undesired gap displacements, it is advantageous if one of the two transporting speeds is the nominal speed of the sandwich belt system, and the coefficient of friction of the transporting means driven at the nominal speed is greater than the coefficient of friction of the transporting means driven at the speed which differs therefrom.

[0017] In order to avoid the items of mail being subjected to additional stressing during transfer to the subsequent segment of the sandwich belt system, the speed which differs from the nominal speed is changed to the nominal speed again prior to transfer.

[0018] It is possible, downstream of the transporting segment, to determine the leading front edges directly by means of a further light barrier or indirectly from the signals of the light barrier arranged upstream of the transporting segment for item displacement, with the aid of the section cycle time.

[0019] In order for the displacement process to take place with a low level of disruption even in the case of items of mail with different lengths, in an advantageous configuration, the transporting segment for item displacement, on each item side, has a circulating sandwich belt which is separate from the rest of the sandwich belt system, is driven in a controlled manner and has a dedicated drive. The length of the displacing sandwich belt region and its spacings from the feeding and discharging sandwich belts of the sandwich belt system are of such a magnitude that, and the accelerating and braking operations are started and ended such that, in the case of overlap, the action-specific length corresponds to the length of the shortest item of mail and the items of mail in the length range which is to be processed are not gripped in the feeding and discharging sandwich belts during their displacement.

[0020] It is advantageous here for the control device to be configured such that an accelerating phase is only started when the first detected edge following the front edge has just left the final clamping location of the transporting segment for item displacement, as seen in the transporting direction, and that a braking phase is ended before the rear edge of the shortest item of mail which is to be displaced has reached the first clamping location, as seen in the transporting direction.

[0021] The invention is explained in more detail hereinbelow by way of an exemplary embodiment and with reference to the drawings, in which:

[0022]FIG. 1 shows a plan view of a sandwich belt system with a transporting segment for item displacement; and

[0023]FIGS. 2 and 3 show an illustration of different overlap variants with an indication as to which item of mail is displaced.

[0024] The items of mail are normally conveyed individually one after the other, by a feeding part 3 of the sandwich belt system, to the transporting segment for item displacement and transported away by a discharging part 4. The deflecting rollers of the sandwich belts are offset in relation to one another such that the items of mail are guided, virtually without interruption, at least on one side and the final clamping points K1, K2 and K3, K4 are as far as possible from one another in order to provide the necessary distances for accelerating or braking purposes without the items of mail still being clamped in the feeding part 3 of the sandwich belt system. Located in the rear segment of the feeding part is a light barrier Li and, on both sides of the transporting path, a measuring means La1, La2 for detecting edges. Correspondingly, at the beginning of the discharging part 4 of the sandwich belt system, measuring means La3, La4 for detecting edges are likewise arranged on both sides of the transporting path.

[0025] The transporting segment for item displacement comprises two endless sandwich belts which are driven separately via deflecting rollers 1, 2 and by means of which the items of mail are clamped in as they are transported. Since all the deflecting rollers are offset in relation to one another, they may be mounted in a rigid manner, even in the case of items of mail of different thicknesses, without jamming taking place. The beginning and end of the displacement segment are defined by the clamping locations K2 and K3, i.e. by first clamping at the rear, incoming deflecting roller 2 and final clamping at the front, outgoing deflecting roller. The nominal transporting speed of the sandwich belt system in the feeding and discharging parts has the value V0, the sandwich belts with the deflecting roller 1 (in the side view the rear sandwich belt) runs at the speed V1, and the sandwich belt with the deflecting roller 2 runs at the speed V2.

[0026] If, on account of a double withdrawal, two overlapped items of mail (overlap) are located in the feeding part 3 of the sandwich belt system, then, first of all, the light barrier Li, which is spaced apart from the first clamping point by a distance which is greater than the length of the longest item of mail, detects the leading front edge (the front edge of the leading overlapped item of mail). Located just alongside this are the measuring means La1, La2 for edge detection. Before the overlapped items of mail are thus gripped in the transporting segment for item displacement, the gaps in relation to the preceding and subsequent item of mail, the item length and all the edges are known.

[0027] It is thus possible to establish which of the two sandwich belts of the transporting segment for displacement purposes has to run at which speed during passage of these overlapped items of mail in order that the items of mail are displaced in relation to one another such that the displacements can be identified, but the gaps in relation to the preceding and subsequent items of mail do not fall below a minimum value which is necessary for disruption-free deflection in subsequent diverter arrangements.

[0028]FIGS. 2 and 3 illustrate examples of displacement, depending on the size and mutual positions, of overlapped items of mail, the level of the differential speed in relation to the nominal speed, for the sake of clarity, not being stated.

[0029] In FIG. 2, the rear items of mail, as seen by the observer, which are driven at the speed V1, are partially overlapped by the front items of mail.

[0030] First of all, a) shows the special case where two items of mail of the same length are overlapped without any lateral offset. One of the two sandwich belts is driven at the nominal speed V0 (V1) and the other sandwich belt is driven at the speed V2=ΔV+V0, with the result that, following forward displacement, an additional edge is measured and a double withdrawal is thus identified. It is not important in this case which of the two sandwich belts is driven at the speed which differs from V0. The forward displacement takes place provided that the gap in relation to the preceding item of mail, despite the displacement, does not fall below the minimum value. In the case of critical values, the differential speed ΔV is reduced.

[0031] A further special case is given under b):

[0032] The items of mail are very long and at the limit of the permitted length agreed. As a result of the partial overlap, the light barrier Li determines an item length above the permitted length, which can only be produced by an overlap. For this reason, displacement of the items of mail in relation to one another is not necessary and the two sandwich belts of the transporting segment for item displacement are driven at the nominal speed V0. In case c), the small item of mail at the front, which has its front edge set back some way in relation to the larger item of mail at the rear, is displaced forward since the gap in relation to the preceding item of mail is large enough. If this gap were in the vicinity of the minimum gap, then the speed V2 of the front item of mail would be decreased to a reduced value V2=V0−ΔV. The item of mail was displaced forward first of all since the length of the small item of mail is not in fact known.

[0033] In the final two examples of FIG. 2, forward displacement of the front, small item of mail in the transporting direction is selected since the gaps are not reduced as a result.

[0034]FIG. 3 gives examples in which the rear items of mail, as seen by the observer, are smaller than the front items of mail and are partially or wholly covered by the latter.

[0035] a) gives a special case corresponding to the example b) of FIG. 2, in which, on account of the maximum length being exceeded, an overlap is identified, with the result that displacement is not necessary.

[0036] In the example b), the rear, concealed letter is displaced forward by the rear sandwich belt at the speed V1=V0+ΔV, since the front gap is sufficiently large. In the case of a small gap, the concealed item of mail would be displaced rearward by a reduced speed V1=V0−ΔV of the rear sandwich belt. In the final examples c) and d), the rear items of mail are displaced forward by an increase in the speed of the rear sandwich belt to V1=V0+ΔV, since the gaps are maintained or increased as a result.

[0037] If, for example, the rear sandwich belt, as seen by the observer, at the speed V1 has a higher coefficient of friction, in order to maintain defined gaps in the case of individual withdrawals, the speed V1=V0 is maintained and the speed of the front sandwich belt V2 is increased or decreased depending on the size of the items of mail, their position in relation to one another size of the gaps. In order for the items of mail to be transported gently, the speed V2 is reduced or increased to V0 prior to the transfer of the item or items of mail to the discharging part 4 of the sandwich belt system.

[0038] At the beginning of the discharging part 4 of the sandwich belt system, the edges are once again detected on both sides by the measuring means La3, La4 for edge detection, as are the spacings of these edges in relation to the formally leading front edge. If there is a deviation from the previous measurement, or if an additional edge is registered, then an overlap is present. In this example, the position of the originally leading front edge is determined indirectly via the transporting speed and the section cycle time. 

1. A method of detecting overlaps in the case of flat items of mail which are transported one behind the other in a sandwich belt system, having at least one light barrier [(Li)] for detecting front and rear edges of the items of mail moving past it, item lengths and gaps being determined from the measurement results from said light barrier, characterized by the following steps: detecting edges on both sides of the transporting path and determining the spacings of the detected edges from the respective leading front edge measured by the light barrier [(Li)], following the edge detection, subjecting the free longitudinal sides of the separated items of mail to frictional force, with different transporting speeds on both sides, in a transporting segment for item displacement, the selection of the differing transporting speeds taking place such that, in the case of overlap, the items of mail would be displaced in relation to one another such that the gap dimensions do not fall below a minimum value, and the action-specific lengths and points in time being selected such that only ever one item of mail is gripped on each side, determining the edges on both item sides, the leading front edges and the spacings of the edges from the respective leading front edge once again following the displacement of the items of mail in relation to one another and detection of an overlap if at least one spacing between the mutually corresponding edges and the respective leading front edge or additional edges has changed in relation to the first measurements.
 2. The method as claimed in claim 1, characterized in that one of the two transporting speeds in the transporting segment for item displacement is the nominal speed of the sandwich belt system, and the coefficient of friction of the transporting means driven at the nominal speed is greater than the coefficient of friction of the transporting means driven at a speed which differs therefrom.
 3. The method as claimed in claim 1, characterized in that, in order to transfer the item or items of mail to the subsequent section of the sandwich belt system, the speed which differs from the nominal speed is changed to the nominal speed.
 4. The method as claimed in claim 1, characterized in that, downstream of the transporting segment for item displacement, the leading front edges are detected by means of a further light barrier.
 5. The method as claimed in claim 1, characterized in that, downstream of the transporting segment for item displacement, the leading front edges are determined, at a certain location, indirectly from the signals of the light barrier arranged upstream of the transporting segment, with the aid of the known transporting speeds and of the section cycle time.
 6. An arrangement for detecting overlaps in the case of flat items of mail which are transported one behind the other in a sandwich belt system, having at least one light barrier [(Li)] for detecting front and rear edges of the items of mail moving past it, item lengths and gaps being determined from the measurement results from said light barrier, characterized by a transporting segment for item displacement in the sandwich belt system with independently activable transporting means which subject the free longitudinal sides of the separated items of mail to frictional force with different transporting speeds on both sides, the action-specific lengths and points in time being selected such that only ever one item of mail is gripped on each side, measuring means [(La1 to La4)] which are arranged on both sides of the transporting path and are intended for detecting edges upstream and downstream of the transporting segment for item displacement and for determining the spacings of the detected edges from the respective leading front edge, a control device for activating the transporting means of the transporting segment for item displacement in dependence on the measured edges on both sides of the transporting path, on their spacings from the respective leading front edges and on the respective gaps in relation to the adjacent items of mail, such that the different speeds are of such a magnitude that the gap dimensions do not fall below a minimum value following displacement, an evaluation device in which the measurements taken upstream and downstream of the transporting segment for item displacement for the spacings between the mutually corresponding edges on both sides of the transporting path and the respective leading front edge are compared and, in the case of deviations or additional edges, an overlap is detected.
 7. The arrangement as claimed in claim 6, characterized in that one of the two transporting means of the transporting segment for item displacement is driven at the nominal speed of the sandwich belt system, and in that the coefficient of friction of the transporting means driven at the nominal speed is greater than the coefficient of friction of the transporting means driven at a speed which differs therefrom.
 8. The arrangement as claimed in claim 7, characterized in that the transporting means in the transporting segment for item displacement at the speed which differs from the nominal speed is changed to the nominal speed again prior to transfer of the respective item or items of mail to the subsequent, discharging segment of the sandwich belt system.
 9. The arrangement as claimed in claim 8, characterized in that the transporting segment for item displacement, on each item side, has a circulating sandwich belt which is separate from the rest of the sandwich belt system, is driven in a controlled manner and has a dedicated drive, and the length of the displacing sandwich belt region, its spacings from the feeding and discharging sandwich belts of the sandwich belt system are of such a magnitude that, and the accelerating and braking operations are started and ended such that, in the case of overlap, the action-specific length corresponds to the length of the shortest item of mail and the items of mail in the length range which is to be processed are not gripped in the feeding and discharging sandwich belts during their displacement.
 10. The arrangement as claimed in claim 9, characterized in that the control device is configured such that an accelerating phase is only started when the first detected edge following the front edge has just left the final clamping location [(K3)] of the transporting segment for item displacement, as seen in the transporting direction, and in that a braking phase is ended before the rear edge of the shortest item of mail which is to be displaced has reached the first clamping location [(K2)], as seen in the transporting direction. 